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Ethernet Calculations

 

100,000,000 bits per second = 1 / 100,000,000 = 0.000 000 010 seconds or 10ns (nanoseconds) per bit.
Max Ethernet frame = 1518 bytes.
Max byte-time on the wire occupied by a max frame = 1518 bytes + 8 bytes Preamble + 12 bytes Inter Frame Gap = 1538 bytes.
1538 bytes per max frame x 8 bits per byte = 12,304 bits per max frame.
12,304 bits per max frame x 10 nanoseconds per bit = 0.000 123 seconds or 123us (microseconds).
Therefore, it takes 123us to output a maximum sized Ethernet frame on Fast-E.
This is the "serialization delay" for Fast-E.

 

100,000,000 bits per second  / 8 bit per Byte = 12,500,000 B/s x 85% Network Utilization = 10,625,000B/s

1518B frame + 8B Preamble + 12B IFG = 1538B max frame

10,625,000B/s / 1538B/b = 6908 max-size frames/second

Minimum Ethernet frame = 64B

64B frame + 8B Preamble + 12B IFG = 85B min frame

10,625,000B/s / 84B/f = 126,488 min-size frames/second

 

Which is more important - bytes or frames? From the calculations above, you can see there are more minimum-sized frames than maximum-sized frames at 85% network utilization, but they are both moving the same amount of data - 10.625MB/s.

If you work out the percentage of InterFrame Gap between frames, there are a lot more IFGs with smaller frames than with large frames. Ultimately, this will lead to a higher maximum utilization of the network segment as the large frames are more efficient.

 

c = speed of light = 186,000 miles per second = 300,000 kilometers per second = 300 million meters per second = 300Mm/s.

T = Thicknet. Propagation velocity of a signal in T is .77c = 231Mm/s.

t = thinnet. Propagation velocity = .65c = 195Mm/s.

UTP = unshielded twisted pair. Propagation velocity = .59c = 177Mm/s.

AUI=attachment unit interface cable. Propagation velocity = .65c = 195Mm/s.

mmf = multi-mode fiber. Propagation velocity = .66c = 198Mm/s.

smf = single-mode fiber. Propagation velocity = .98c = Mm/s.

 

Round Trip Time, RTT, for UTP = distance between sites / 177 Mm/s x 2

 

Repeater Rules

Four 10Mb/s repeaters permitted in series. Exceeding number of repeaters and maximum combined cable lengths can lead to signal reduction, lost preamble bits leading to sync problems, late collisions.

Two 100Mb/s Type II repeaters permitted in series. Exceeding number of repeaters can cause analog-to-digital conversion issues leading to sync problems.

 

Autonegotiation Challenges

Set your switches to fixed speed and fixed duplex.

Users will connect using autonegotiate.

Users experiencing troubles connecting will need to have their configurations changed to fixed/fixed.

 

Gigabit Autonegotiation

Both ends should send 4 copies of their requests and acknowledge 4 copies of their peer.

If either end fails to ack 4 times, the end noticing the deficiency may force an autonegotiation restart.

When in doubt, use a tap in between the communicating ends and attach your Gig-E analyzer to the tap.

While waiting for the vendor to fix the autonegotiation problem reported with your Gig-E analyzer in 10-bit mode, set both ends to fixed/fixed.

 

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